\input pstfig
$
\pnode(0,	2	){A}
\pnode(2.5,	3	){B}
\pnode(5.5,	0.3	){C}
\pnode(0.5,	0	){D}
\pcline(A)(B)
\pcline(B)(C)
\pcline(C)(D)
\pcline(D)(A)
\thinline\dashline
\pcline(A)(C)
\nLput[offset=-1.5ex](A)(C){0.55}{d_2}
\pcline(B)(D)
\nLput[offset=1.5ex](B)(D){0.3}{d_1}
\psIntersectionPoint(A)(C)(B)(D){P}
\setLCNode(C){1}(P){-1}{PC}
\setLCNode(D){1}(P){-1}{PD}
\solidline
\psarc{<->}(P){0.6}{(PD)}{(PC)}
\nput[labelsep=0.8ex]{-70}{P}{\varphi}
$
\bye
